April Kirkwood says she lost her virginity to Frankie Valli at the age of 16 — when he was 39.Getty Images (left)

April Kirkwood’s decades-long love affair with Frankie Valli began when she was just 6 years old. As she remembers it, she loved The Four Seasons more than anything, even at that age, and her mother and aunt would take young April to shows, talking their way backstage. April would make a beeline for Valli, and he’d scoop her up and carry her around while she nuzzled her face in his neck.

Ten years later, she’d be losing her virginity to him at a Holiday Inn.

April Kirkwood with Frankie Valli at the age of 8

“He was 23 years older than me,” Kirkwood says today — 39 to her 16. “That first night was sad. I had never seen a naked man before. I was like, ‘Oh, my God. What did I just do?’”

April Kirkwood at age 12 with Frankie Valli

That night would set the tone for her romance with the frontman for The Four Seasons — in the 1960s, until The Beatles, the most popular band in the world. “For me, there was nothing else,” writes Kirkwood, in her new memoir, “Big Girls Do Cry: My Love Affair with Frankie Valli” (Wise Ink Creative Publishing).

April Kirkwood at age 41 with Frankie Valli

After having sex with Valli that first time, the singer said, “Now don’t do this with anyone else.” He added that it was late and she should get going home.

“I was smart enough to go, ‘Can I have your phone number?’ ” Kirkwood says. “And, of course, he gave me his manager’s number.”

Kirkwood drove home a little after midnight, her mother waiting up. “Did you have a good time?” she asked. “Did you see Frankie at all?”

“Yeah, Mom, thanks for letting me go. Night. Love you.”

Not long after, while still attending high school in her native Youngstown, Ohio, Kirkwood asked her mother to drive her to Akron, an hour away, where Valli had summoned her to get on the tour bus. Her mother promptly put her on birth control.

“I’ll never forget what my mother said to him that day: ‘Take care of my daughter,’ ” Kirkwood writes. “He respectfully shook her hand.”

Kirkwood thinks her mother had the misguided notion that Valli was her daughter’s ticket out of a tiny Ohio town, off the chicken farm on which they were living with Kirkwood’s grandmother and several aunts. Her mother had been abused as a girl and suffered from mental illness; Kirkwood’s parents had divorced when she was 4.

“My mother was so twisted in her own pain that she would’ve done anything to get me out of there,” Kirkwood says. So April joined the tour for a few days. “It never crossed my mind that I was his Ohio groupie, his hookup, his lay on the road,” she writes, but there she was, stashed on a cramped tour bus with no doors, only drapes, expected to have sex with Valli while his bandmates and crew listened.

“That was awful,” Kirkwood says today. “The most embarrassing thing.” But she was a teenage girl on the road with one of the most famous men in the world, and she was confused: “Sex with a celebrity — what’s the protocol?” she asks.

Valli would summon her to get on a bus or a plane — he’d pay, always coach — to meet him in cities such as Akron, Cleveland, Pittsburgh, Syracuse, in Holiday Inns or sometimes Hiltons. She never questioned why he didn’t fly her to San Francisco or Chicago or New York City.

“He was so glamorous to me, I think I could’ve met up with him at McDonald’s,” Kirkwood says. He was uneducated and unsophisticated, but she adored everything about him: the way he put french fries on top of his burger and smashed it all between the buns; the gold chains and velvet suits; his diktats that she stay beautiful and never talk too much.

“It was a very first-generation Italian-American thing,” she says. “If you can’t sleep with it, eat it or drink it, it’s no good to me.”

She went back to high school and dreamt of marrying Valli, even though Valli already had a wife. Then she saw the latest Four Seasons album, a blond model named Mary Ann on the cover. She knew — the next Mrs. Valli had been chosen already.

“That really killed me,” she writes. “I went blond from that moment on.”

April Kirkwood and Frankie Valli

Even after a night together in Syracuse, when Valli lit a cigarette right after sex and casually said, “By the way, I think I’m going to marry my girlfriend,” Kirkwood never gave up hope that she would eventually be the one. She listened to his endless complaints about his wives or girlfriends while the 5-foot-4 singer strutted around the room naked.

“Frankie openly vented about his relationships as if he wanted to come clean somehow,” Kirkwood writes. “He vented about what he said to Mary Ann when she wrecked his car. I could sense that Frankie was still fuming as he recounted how he instructed the maid to get Mrs. Valli on the phone . . . Breathless and stunned, I always sat on the side of the bed wrapped in sheets, knowing that once again I was considered only Ms. Almost Good Enough. This had become a recurring nightmare upon returning home after brief stays with this legend.”

In 1975, Valli — his star fading, The Four Seasons culturally irrelevant to a new generation electrified by Dylan and Hendrix and The Beatles and The Doors — sat for an interview with People magazine and spoke about his new wife, Mary Ann Hannagan. She was 20 years old when they met.

“She was a pal when I needed one,” Valli told the magazine. “But because I’d been hurt before, I resisted what I was feeling about her.”

For her part, Kirkwood searched for men who reminded her of Valli, and she married and divorced twice — men she calls her “substitutes for Mr. Valli.”

He was the love of my life, and I don’t think he ever cared for me.

- April Kirkwood

Neither husband had a chance. Kirkwood thought only of Valli and what she could do to convince him she was worthy. “I did anything — not just sexually — that he wanted, because deep in my heart, I believed I would one day be the next Mrs. Frankie Valli,” she writes.

In 1980, Valli suffered two devastating losses: His stepdaughter ­Celia died after slipping on a fire escape, and six months later, his youngest, Francine, succumbed to a drug overdose.

Kirkwood was 19 at the time, and had been due to hook up with Valli in Hawaii. He disappeared from her life.

Five years passed. Kirkwood was living with Frankie Valli replacement No. 2, a man named Billy, when the singer himself summoned her to Chicago, out of the blue. She was elated: She went to Saks and bought dresses and lingerie that she couldn’t afford and headed off to the airport, giddily flying coach from Ohio.

She spent two post-show nights with Valli until her boyfriend showed up. Kirkwood was furious — as was Valli, who sent her packing. She cried the entire flight home and felt more trapped than ever: Here she was in a tiny apartment in a suburb of Ohio, living with a boy of 18, convinced beyond all reason that he had ­ruined her best chance at becoming the rich, respected wife of an icon.

A few days later, the phone rang. It was Valli. “Work on your marriage with your husband,” he told her. “Maybe we will talk again soon.”

Kirkwood knew she’d never hear from him again and went into a deep depression. She spent days unable to get out of bed.

“If I’d been committed to my career as I was to Frankie, I would be running my own private Ivy League school by now,” she writes. Kirkwood eventually settled for Billy, marrying and having two children before divorcing him nearly 20 years later.

Her certainty that she’d never see Valli again was unfounded. Nearly 15 years later, in 1995, she resumed hooking up with him on the road. Little had changed. After sex, Valli would complain to Kirkwood about his latest wife, Randy, who was 26 years young­er. She told him about her newest boyfriend, Ron — another Valli doppelganger, this one married.

“April, if he’s seeing you, he probably has other girlfriends, too,” Valli told her. “And if by some slim chance he does leave his wife, you will become the wife and someone will replace you.”

Kirkwood couldn’t believe the hypocrisy. Yet she quickly looked past it — just as she looked past all the things she heard from his entourage about Valli’s issues with food, an eating disorder that had him bingeing on maple syrup. She looked past the night Valli couldn’t find her and instead tried to sleep with her aunt. She looked past the time she walked by his dressing room and caught him making out with a backup singer.

She’d gloat of her “friendship” with Valli to people who knew him, only to hear, “Oh yeah? He’s a heck of a performer, a mean ­human being.”

“Everybody thought he was ­arrogant,” Kirkwood says. “I think he is a very, very, very selfish ­person.”

Still, she couldn’t escape him — when “Jersey Boys” debuted on Broadway, she went, tears streaming down her face the ­entire time. “This is my story!” she says. “Even though I’m not in it. This is my truth — innuendo about girls on the road. Or about one of his wives. What killed me is that I’d been there for the whole thing.”

One of her last encounters with Valli was in 2005 — as always, backstage, after a show. She had tracked down his email address after an airline stewardess sold the story of her affair with Valli to the National Enquirer. Kirkwood had written something like, “See? At least I never sold you out.”

Unable to stay away, Kirkwood tried again. On Thursday night, Valli was playing a show in her Ohio neighborhood at a venue called The Lake Club. She’d bought a $995 Herve Leger dress, two tickets (one for herself, one for Ron), and arrived at the venue three hours before showtime, clutching a copy of her book. She went looking Valli.

“I talked to someone from Frankie’s camp and he goes, ‘Oh. You’re her,’ ” Kirkwood says. She tried to give him the book, which she’d inscribed.

“No f–king way,” he said. “I’m not touching this.”

She spent the next few hours asking waitresses and staff where she could find Valli, and when she got close, “eight bodyguards came out,” she says. They also ­refused to take the book.

Kirkwood and her boyfriend took their seats in the fifth row, Kirkwood sobbing her way through the entire set. She got drunk, and her boyfriend took her home and put her to bed.

“I’m heartbroken,” she says. “Personally, I think Frankie is a motherf–ker. He was the love of my life, and I don’t think he ever cared for me.”

Though she’s said it before, this time, Kirkwood, now 57, says she really means it: “I am done. I am so done.”